"previous caution, prudent foresight (to prevent mischief or secure good results); a measure taken beforehand, an act of foresight," c. 1600, from French précaution (16c.) and directly from Late Latin praecautionem (nominative praecautio) "a safeguarding," noun of action from past-participle stem of Latin praecavere "to guard against beforehand," from prae "before" (see pre-) + cavere "to be one's own guard" (see caution (n.)). In mid-20c. a euphemism for "contraception." The verb meaning "to warn (someone) in advance" is from c. 1700.
word-forming element making adjectives from nouns, meaning "having, full of, having to do with, doing, inclined to," from Old French -ous, -eux, from Latin -osus (compare -ose (1)). In chemistry, "having a lower valence than forms expressed in -ic."